ACTIVE NUCLEAR-PORE COMPLEXES IN CHIRONOMUS - VISUALIZATION OF TRANSPORTER CONFIGURATIONS RELATED TO MRNP EXPORT

The Nuclear Pore Complex (NPC) regulates nucleocytoplasmic transport b y providing small channels for passive diffusion and multiple docking surfaces that lead to a central translocation channel for active trans port, In this study we have investigated by high resolution scanning a nd transmission electron microscopy the dynamics of NPC structure in s alivary gland nuclei from Chironomus during Balbiani ring (BR) mRNP tr anslocation, and present evidence of rearrangement of the transporter related to mRNP export, Analysis of the individual NPC components veri fied a strong evolutionary conservation of NPC structure between verte brates and invertebrates. The transporter is an integral part of the N PC and is composed of a central short double cylinder that is retained within the inner spoke ring, and two peripheral globular assemblies w hich are tethered to the cytoplasmic and nucleoplasmic coaxial rings b y eight conserved internal ring filaments. Distinct stages of BR mRNP nuclear export through the individual NPC components were directly vis ualized and placed in a linear transport sequence, The BR mRNP first b inds to the NPC basket, which forms an expanded distal basket ring, In this communication we present stages of BR mRNP transport through the nucleoplasmic, central and cytoplasmic transporter subunits, which ch ange their conformation during mRNP translocation, and the emegence of mRNP into the cytoplasm. We propose that the reorganization of the ba sket may be driven, in part, by an active translocation process at the transporter. Furthermore, the images provide dramatic evidence that t he transporter functions as a central translocation channel with trans iently open discrete gates in its globular assemblies, A model of NPC transporter reorganization accompanied with mRNP translocation is disc ussed.